Energy sources for aquatic animals in the Orinoco River floodplain: evidence from stable isotopes.

Stable carbon and nitrogen isotope ratios in autotrophs, aquatic invertebrates and fishes from the Orinoco River floodplain of Venezuela reveal that microalgae, including both phytoplankton and epiphytic (attached) forms, are predominant energy sources for many aquatic animals, even though aquatic vascular plants are much more abundant. Floating mats of the grass Paspalum repens and the water hyacinth Eichhornia spp. harbor particularly high densities of aquatic animals, but isotopic evidence indicates that few species are dependent on organic carbon originating from these plants. The stable isotopic evidence for the trophic importance of algae contradicts traditional interpretations of food webs in freshwater wetlands, which are generally thought to be based largely on detritus originating from vascular plants.